Catalytic Construction of Carbon-Heteroatom Bonds by Shining Visible-Light on Transition-Metal Photocatalysts

Aspects of sustainability are very crucial for the development of new synthetic methods. In this context, visible-light has been used as an abundant source of energy to induce or activate chemical reactions. Photoredox catalysis provides the opportunity to generate highly reactive radical ion intermediates often with unusual or unconventional reactivities under mild reaction conditions. Ru(II) or Ir(III)-polypyridyl complexes and organic dye sensitizers have been the widely used chromophores owing to strong absorption in the visible region, long excited-state lifetimes, and high redox potentials. However, the cost and adverse environmental impact of these heavy transition-metal complexes, as well as their restrictive conformational stability (both with respect to inner-sphere substrate interactions and as consequence asymmetric transformations), limit their applications. Given the crucial need to develop environmentally benign, cheap, multi-purpose, and flexible catalytic systems, copper has emerged as an appealing alternative. Copper-based photocatalysts display highly tunable redox properties in their excited states. Moreover, the combination of conventional photocatalysts with copper(I) or copper(II) salts has emerged as an efficient dual catalytic system for many cross-coupling reactions. This thesis demonstrates the use of both ruthenium and copper-based photocatalysts in visible-light mediated transformations. Firstly, a visible-light mediated photocatalyzed method for the synthesis of substituted pyrazines from vinyl azides has been developed. This method utilizes a dual energy and electron transfer strategy from the photoexcited catalyst. This method has a broad substrate scope and does not require any additives, moreover, it operates at room temperature which is also advantageous in terms of handling azides. Then a visible-light-accelerated, copper(II)-catalyzed method for oxo-azidation of vinylarenes has been developed which operates at room temperature and utilizes molecular oxygen as the stoichiometric oxidant. In contrast to commonly used iridium, ruthenium or organic dye-based photocatalysts, copper-based photocatalysts were found to be unique for this transformation, which is attributed to the inner-sphere mechanism. With spectroscopic evidence, Cu(II) has been proposed as the catalytically active species. In the key-step, a copper-azide species undergoes a light-accelerated homolysis to form Cu(I) and azido radicals. This represents the first visible-light photocatalyzed process triggered by copper in the oxidation state +2. Lastly, chloro-sulfonation of unactivated olefins has been developed utilizing commercially available sulfonyl chlorides and copper-phenanthroline-based photocatalysts. Besides the Cu(I) complex [Cu(dap)2]Cl, the corresponding Cu(II) complex [Cu(dap)Cl2] proved to be an efficient catalyst in this reaction, being advantageous from an economic point of view but also opening up new avenues for photoredox catalysis. Moreover, these copper complexes outperformed commonly used ruthenium, iridium, or organic dye-based photocatalysts, owing to their ability to stabilize or interact with transient radicals by inner-sphere mechanisms due to the persistent radical effect. The use of stoichiometric Na2CO3 in combination with the copper photocatalysts was found to be essential for this reaction. As suggested by appropriate control experiments, the role of Na2CO3 is attributed to the prevention of poisoning of the catalyst